1. Field of the Invention
This application claims priority under 35 U.S.C. .sctn. 119 and/or 365 to "Engine Balancer Unit," Application No. H09307521 filed in JAPAN on Nov. 10, 1997, the entire content of which is herein incorporated by reference.
This invention relates to an engine balancer unit.
2. Description of the Related Art
In an engine for vehicle, a crankshaft may be unbalanced due to, for example, the secondary inertial force generated by reciprocal movement of the piston. Such an unbalance causes vibration of the engine. In order to reduce such unbalance, a conventional engine incorporates two counterbalance shafts which includes balance weights eccentrically provided from the axis of the counterbalance shafts. The counterbalance shafts are rotated by the crank shaft in the opposite direction. Further, the counterbalance shafts are supported by a unitary case for easy installation to the engine. Such conventional engine balancer unit are disclosed in Japanese Laid-Open utility Model Patent Publication No. 5820750 published on Feb. 8, 1983 or Japanese Utility Model Patent Publication No. 58-36912 published on Aug. 19, 1983.
As shown in FIGS. 6 and 7, in Publication No. 5820750 a crankshaft 602 is supported by a shaft support 603 in a cylinder block 601. The shaft support 603 is mounted on an intermediate wall member 604 disposed in the cylinder block 601. Two counterbalance shafts 612 are supported by a unitary case 609 which is coupled to the intermediate wall members 604 in a detachable manner. The counterbalance shafts 612 are rotated by the crank shaft 602 through gears 606 and 614.
In this conventional design, for ease of installation to the engine, the balance weights are mounted on two counterbalance shafts 612 that are supported by the unitary case 609. However, the cylinder block 601 must be modified to create boss portions that become the intermediate wall members 604. Recently, standardization of the cylinder block has become important for low cost mass-production of engines. Enormous plant investment becomes necessary to change the manufacturing line of the cylinder block 601 to carry the engine balancer unit, so that engine manufacturing costs are greatly increased.
As shown in FIG. 8, in Publication No. 58-36912, two counterbalance shafts 804a and 804b are supported by a unitary case 805 which is held between a cylinder block 801 and an oil pan 802. The counterbalance shafts 804a and 804b are rotated by the crankshaft 808 through gears 807, 806a and 806b.
In this conventional design, the cylinder block 801 does not need to be modified to carry the engine balancer unit. However, engine oil may leak from either the junction with the cylinder block 801 or that with the oil pan 802. In other words, it is complicated to maintain fluid tightness at more than one junction with conventional fasteners. Further, the oil pan 802 may interfere with road surface obstacles, and the base part of the oil pan is likely to fail since the oil pan 802 extends downward by the thickness of the unitary case 805 which is inserted between the cylinder block 801 and the oil pan 802.